1. Field of the Invention
The present invention relates to methods of displaying signal quality in digital data transmission systems in which inter-symbol interference (ISI) is present and cannot be practically removed by passing the signal through a filter. More specifically, the present invention relates to the display of signal quality in PSK transmission systems having ISI, such as GSM-EDGE.
2. Art Background
In the development, deployment, and operation of signal transmission systems, instruments are needed which will display signal quality, and assist the operator in identifying problems in the system. This is particularly true of digital systems. In systems using Phase Shift Keying (PSK), data is encoded by varying carrier phase and magnitude. Commonly, PSK schemes are identified by the number of different phase angles used in transmission. A scheme such as 8-PSK uses 8 equally spaced carrier phase angles. Each of these individual phase angles is called a symbol, and the set of symbols taken together is called a constellation.
In an ideal PSK transmission and reception system, a graphical display of symbols in I-Q space would show a discrete set of points on a circle. In such a display, it is easy to observe transmitter problems such as gain imbalance, phase noise, or the presence of spurious tones. Such instruments are typically referred to as Error-Vector-Magnitude (EVM) instruments.
When transmitting symbol streams on the crowded radio spectrum, filters are commonly used to limit the bandwidth of the transmitted signal. A side effect of these filters is that in limiting or shaping signal bandwidth, the signal is distorted. The nature of the distortion depends on the particular sequence of symbols; in decoding a particular symbol, the signal representing that symbol is distorted by the effect of the filter on the symbol and the symbols which surround it. Because this distortion is symbol dependent, it is called inter-symbol interference (ISI). In many systems, ISI introduced by a transmit filter is removed by a receive filter. In other systems, ISI cannot be practically removed because the bandwidth of the signal is too narrow to allow for zero ISI conditions.
GSM (originally Groupe Spxc3xa9cial Mobile, now known as Global System forMobile Communications) is such a digital system in world wide use. GSM-EDGE represents an extension of GSM technology which allows for increased data rates in the same spectrum by changing the modulation from GMS""s constant envelope GMSK 8-PSK to a system using 3xcfx80/8 8-PSK with symbol rotation, using both phase and amplitude modulation, but using the same sets of GSM filters. While GSM-EDGE offers increased data rates, it also suffers from significant in increases inter-symbol interference, as the transmit filter was chosen to produce a spectrum similar to that produced by earlier 0.3 GMSK waveforms, without regard to its ISI properties.
This inter-symbol interference has the effect of making a raw constellation display very difficult to use as an indicator of signal quality. What is needed is a method of displaying signal quality in systems having inherent inter-symbol interference.
Signal quality in an input signal containing inter-symbol interference is displayed by modeling the signal as comprising an ideal signal component plus an ISI component plus an error component, and subtracting out the ISI, leaving the ideal signal plus the error. In a first embodiment, the signal component is recovered from the input signal through demodulation. The recovered bit sequence is used to generate a correction signal equal but opposite to the ISI. The correction signal is added to the input signal in a feedforward fashion, resulting in the input signal minus ISI, plus any residual errors. The signal with these residual errors is then displayed. In a second embodiment, the error signal is derived from the input signal, and combined with a regenerated ideal signal, producing the signal with error, without ISI.